1. Field of the Invention
The invention relates to data hiding, and more particularly, to data hiding methods and corresponding processing systems for hiding secret data in grayscale (or gray-level) images using a technique with moderate computational complexity.
2. Description of the Related Art
Digital halftoning is a process to display grayscale images with a two-tone texture pattern. Halftoning is mainly used as printouts for materials such as magazines, newspapers, and books, generating a black-and-white format. Halftoning mainly takes advantage of the fact that the human visual system is not highly sensitive, so that black and white pixels of a dense uniform grid may be used to represent a desired grayscale effect. Halftoning techniques can be mainly divided into two categories: single pixel processing, and neighboring pixel processing. For single pixel processing, a halftoning output can be obtained by comparing pixel values of every pixel of an original image with some masks. Ordered dithering, for example, is a well-known scheme in the single pixel processing field.
Recently, transmittance of digital information over the Internet has rapidly grown. Digital data may be easily downloaded or manipulated and intentionally tampered with, thus making the issue of intellectual property protection more significant. Most multimedia files may be stored in a compressed bit stream format to save on storage space or transmission time. As a result, methods for data hiding have grown in significance. Embedding digital watermarks or digital signatures in multimedia content is one method of intellectual property protection for digital information, verification of ownership rights and assuring accuracy of digital information.
Hiding data in grayscale images is commonly used for data hiding. One coding method used to hide data in grayscale images is the Block Truncation coding (BTC) method. In the BTC method, an original image is divided into a number of non-overlapping image blocks and pixel values of all pixels in each non-overlapping image block are replaced by the maximum value and the minimum value of each block. Therefore, only the maximum value, the minimum value of each block and a bitmap corresponding thereto need to be transmitted for an image of high visual quality to be reconstructed at the receiving end, thus speeding up the calculation.
However, the annoying false contour and blocking effect may inherently exist in a BTC image which is encoded utilizing the BTC method, due to each block of the encoded BTC image only contains the maximum value and the minimum value of the block. In addition, image quality of the encoded image may rapidly deteriorate to be unacceptable when block truncation size increases, effecting the image quality of the encoded image.